ABSTRACT Aging is associated with a progressive decline of metabolic health and represents a unique risk factor for the development of type 2 diabetes, cardiovascular disease, and non-alcoholic fatty liver disease in the elderly. The biology underlying age-related metabolic disease is likely multifaceted, and conceptually, may involve intrinsic changes in tissue metabolism and perturbations of inter-tissue metabolic crosstalk. Endocrine factors play a critical role in modulating carbohydrate and lipid metabolism and maintaining systemic energy homeostasis. Perturbations of endocrine signaling are commonly observed during mammalian aging. However, the nature of endocrine signals that govern metabolic homeostasis during mammalian aging remains poorly defined. In preliminary studies, we identified Neuregulin 4 (Nrg4) as a novel adipocyte-derived secreted factor that protects mice from insulin resistance and hepatic fat accumulation in an age-dependent manner. The expression of Nrg4 in mouse adipose tissues was elevated by caloric restriction. These findings form the basis for our central hypothesis that endocrine signaling by adipokines plays a uniquely important role in preserving metabolic homeostasis during aging. In this proposal, we plan to evaluate the physiological role of this factor in age-dependent metabolic regulation using gain- and loss-of-function mouse models. We will delineate its regulation during aging and explore the molecular and metabolic mechanisms involved.